The Research Infrastructure (RI) ACTRIS – Aerosols, Clouds and Trace Gases - is the pan-European RI that consolidates activities amongst European partners for observations of aerosols, clouds, and trace gases and for understanding of the related atmospheric processes, to provide RI services to wide user groups. ACTRIS is composed of 8 connected elements: distributed National Facilities (observation platforms and exploratory platforms) both in Europe and globally, and 7 Central Facilities (Head Office, Data Centre and 5 Calibration Centres). ACTRIS provides access to its facilities, open-access data, research support, instrument calibration and development, and training to various user groups. By providing data and access ACTRIS enhances science, but it also generates and disseminates knowledge, boosts technological development, and creates human capital and jobs for the benefit of the society. ACTRIS will positively impact on e.g. human health, climate resilience, and protection from environmental hazards and reduction of air pollution. ACTRIS has been selected to the ESFRI roadmap in 2016 as mature enough to be implemented within the next ten years. ACTRIS Preparatory Phase Project (PPP) will have a significant role in enabling the transition from a project-based network of research facilities to a centrally coordinated integrated pan-European RI. ACTRIS PPP brings together a wide community of research performing organizations, research funding organizations and ministries needed to take the decisions and actions to move forward in the implementation of the ACTRIS. The main objectives of ACTRIS PPP are to develop the organizational, operational and strategic frameworks of the RI. The work includes legal, governance, financial, technical, strategic, and administrative aspects carried out in 9 work packages. The main outcomes of PPP are signature-ready documents for establishment of a legal entity with well-defined operations and a sound business plan.